Radiographic apparatus

ABSTRACT

For Respiration Correlated Cone Beam CT scanning, we have observed that improvements in the frame rate are in fact undesirable. We therefore propose a radiographic apparatus comprising a beam of radiation and a detector therefor, adapted to obtain a two dimensional image of the beam after passing through a cyclically varying object to be investigated, a processor adapted to review the images and select images at like points in the cycle, and a control means for the beam of radiation adapted to activate the beam periodically. The control means can activate the beam at a frequency of between 0.5 and 5 Hertz, more preferably between 1 and 3 Hertz, which corresponds (roughly) to a frequency that is between 6 and 10 times the frequency of the cyclical variation. It will assist if the selected point of the cycle is an extremity thereof, as the rate of change in these areas is at a minimum. Thus, slight mismatches between the two cycles will then have only a small effect. Typically, the object will be a patient and the cyclical variation will be the patient&#39;s breathing cycle.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application ofInternational Application No PCT/EP2006/009801, filed Oct. 11, 2006 andpublished as WO 2008/043378 A1 on Apr. 17, 2008, the content of which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to radiographic apparatus, including suchapparatus when operating alone or in conjunction (for example integratedwith) radiotherapeutic apparatus.

BACKGROUND ART

Cone beam computed tomography (CBCT) scanners are well known and produceuseful images of the interior structure of patients. They are invaluableas a diagnostic tool, and can also be used in conjunction withradiotherapeutic apparatus to produce realtime positional verificationof organ location and even realtime guidance of the therapeuticradiation.

Such scanning does however meet with difficulties if the patient is notstill. The three-dimensional tomograph is computed from a number oftwo-dimensional images, and the assumption must be made that the imagesare of an identical structure. If the patient (or parts of the patient)have moved between images then this results in degradation of thetomography and/or image artefacts. Such movement is of courseinevitable, in the form of respiration and cardiac cycles.

Generally, improvements in the apparatus that allow a higher frame rateare regarded as desirable. These allow more images to be collected in ashorter time, resulting in an improved three dimensional tomographyand/or reduced time demands on the patient.

To overcome the issue of respiration artifacts, we have proposed CBCTscanning that is correlated with the respiration cycle. This can be doneeither by detecting the respiration cycle and gating the scanneraccordingly, or by scanning the patient and ascertaining the cyclicalphase of a specific image from the image content. WO2004/06464 andWO2004/066211 describe such systems and a suitable algorithm fordetermining the phase of a specific image. This allows images of the“wrong” phase to be discarded prior to computation. Such respirationcorrelated CBCT (RCCBCT) allows good quality images of structures closeto the lungs and/or diaphragm to be obtained.

SUMMARY OF THE INVENTION

For RCCBCT, we have observed that improvements in the frame rate are infact undesirable. Instead of obtaining more images (or the same numbermore quickly), a higher frame rate simply results in a greater number ofimages being discarded by the selection algorithm. This means that thereare no improvements in image quality or in the time required foracquisition, and the patient is exposed to a greater radiation dosewithout any corresponding benefit.

We therefore propose a radiographic apparatus comprising a beam ofradiation and a detector therefor, adapted to obtain a two dimensionalimage of the beam after passing through a cyclically varying object tobe investigated, a processor adapted to review the images and selectimages at like points in the cycle, and a control means for the beam ofradiation adapted to activate the beam periodically.

The control means can activate the beam at a frequency of between 0.5and 5 Hertz, more preferably between 1 and 3 Hertz. This ideallycorresponds (roughly) to a frequency that is between 6 and 10 times thefrequency of the cyclical variation.

It will assist if the selected point of the cycle is an extremitythereof, as the rate of change in these areas is at a minimum. Thus,slight mismatches between the two cycles will then have only a smalleffect.

Typically, the object will be a patient and the cyclical variation willbe the patient's breathing cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures in which;

FIG. 1 is a view of a cone beam CT scanner according to the presentinvention, viewed along the axis of rotation thereof;

FIG. 2 is a schematic view of the system incorporating such a scanner;

FIG. 3 shows a treatment apparatus including the scanner of the presentinvention; and

FIG. 4 shows the effect of phasing the radiation delivery according tothe present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a cone beam CT scanner. A patient 10 is supported on acouch 12 which may be of any suitable design. Couches typically allowthe elevation and longitudinal position of the patient to be adjusted,and this may be provided for as desired.

An x-ray source 14 is arranged to project a wide beam 16 of radiationgenerally directed towards the isocentre 18 of the patient. The source14 is rotatable around the isocentre 18 on a rotational support 20. Thesupport can, for example, be in the form of a ring or annulus around thepatient 10 and couch 12 in which the source is mounted, or it can be aC-arm, or any suitable support allowing the source to rotate, or anycombination thereof.

A two-dimensional flat-panel detector 22 is also mounted on the support20, opposite the source 14 and arranged to rotate in synchronismtherewith. If the support includes a C-arm then this can be achieved bymounting the detector on the opposite arm.

Thus, radiation emitted by the source 14 is partially absorbed by thepatient and the attenuated signal is detected by the flat panel detector22. The source 14 and detector 22 are then indexed rotationally and afresh image obtained. This is repeated until sufficient images areacquired to reconstruct the volume data, typically one completerotation.

FIG. 2 shows the system as a whole. The scanner of FIG. 1 is shown,together with cables linking the source 14, detector 22 and rotationalsupport 20 to a plurality of computing means 24, 26 which process thedata generated including the images, source intensity (etc), androtational support position. Data is output via any suitable means,depicted generally as a monitor 28 but not limited thereto, and thesystem is controlled by any suitable input means, again depictedgenerally as a keyboard 30 but likewise not especially limited thereto.

As mentioned above, we have found that there are artefacts in thereconstructed volume data of cone beam CT systems, which we have tracedto patient breathing movements. To overcome or alleviate these,respiration correlation techniques are applied to the acquiredprojection images by the computing means 24, 26.

To assist in this process, a breath control system is provided at 32 tomonitor the phase of the patients breathing while the projection imagesare acquired. On completion of the acquisition, projection images thathave comparable breathing phases can be selected from the complete set,and these are used to reconstruct the volume data using cone beam CTtechniques. As a result, any phase or range of phases can be selectedand therefore the effect of breathing can be studied if desired.

As an alternative to the breath control system, it is possible to use afeature in the projection image(s) to determine the breathing phase,such as the position of the patient's diaphragm. This can then be usedto select the relevant images to be used in the projection process.

An alert system including a light 34 and a buzzer 36 is provided, toprompt the patient visually and audibly in order to ensure a regularamplitude and pattern of breathing. Other alerts could of course beemployed, such as other forms of visible prompts including (for example)movable devices, and other forms of audible prompts including (forexample) speakers, percussive devices or any other form of controllablesound generation apparatus.

As a further alternative to the breath control system, the images can beanalysed to ascertain their phase and the appropriate images selectedfor use. An example of such analysis is set out in WO2004/066211, thecontent of which is hereby incorporated by reference. The reader isalerted that the disclosure of WO2004/066211 is considered relevant tothis application and may be used as a source of amendments to thisapplication if necessary.

FIG. 3 shows a system including a therapeutic source of radiation 38arranged to emit a suitably collimated beam of therapeutic radiation 40.This allows simultaneous scanning and treatment. If the radiation fromsource 14 continues during the treatment, the output of the radiographicapparatus can be used to control delivery of therapeutic radiation fromthe source 38, dependent on the patient's breathing cycle. This ensuresthat the tumour is in the correct position when the radiation isdelivered.

Such monitoring does of course mean that many images are discarded. Tolimit the dose applied to the patient, therefore, the source 14 ispulsed as shown in FIG. 4. The typical breathing cycle 42 has a periodof about 4 seconds, i.e. a frequency of about 0.25 Hz. A pulse rate of 2Hz therefore produces about 8 scans 44 per breathing cycle. If we(arbitrarily) choose a particular point in the breathing cycle, it canbe seen that an image 46, 48 is obtained close to that point in eachcycle. This applies even though (as shown in FIG. 4) the breathing cycleis only approximately 0.25 Hz and therefore the pulse rate is not anexact multiple of the breathing cycle. Selection of a point in thebreathing cycle corresponding to one of the limits thereof will assistsince the rate of change at this point is less.

It will of course be understood that many variations may be made to theabove-described embodiments without departing from the scope of thepresent invention.

1. A radiographic apparatus comprising a beam of radiation and adetector therefor, adapted to obtain a two dimensional image of the beamafter passing through a cyclically varying object to be investigated, aprocessor adapted to review the images and select images at like pointsin the cycle, and a control means for the beam of radiation adapted toactivate the beam periodically.
 2. The radiographic apparatus accordingto claim 1 in which the control means activates the beam at a frequencyof between 0.5 and 5 Hertz.
 3. The radiographic apparatus according toclaim 1 in which the control means activates the beam at a frequency ofbetween 1 and 3 Hertz.
 4. The radiographic apparatus according to claim1 in which the control means activates the beam at a frequency that isbetween 6 and 10 times the frequency of the cyclical variation.
 5. Theradiographic apparatus according to claim 1 in which the selected pointof the cycle is an extremity thereof.
 6. The radiographic apparatusaccording to claim 1 in which the object is a patient.
 7. Theradiographic apparatus according to claim 6 in which the cyclicalvariation is the patient's breathing cycle.
 8. (canceled)
 9. Theradiographic apparatus according to claim 2 in which the selected pointof the cycle is an extremity thereof.
 10. The radiographic apparatusaccording to claim 3 in which the selected point of the cycle is anextremity thereof.
 11. The radiographic apparatus according to claim 4in which the selected point of the cycle is an extremity thereof. 12.The radiographic apparatus according to claim 2 in which the object is apatient.
 13. The radiographic apparatus according to claim 3 in whichthe object is a patient.
 14. The radiographic apparatus according toclaim 4 in which the object is a patient.
 15. The radiographic apparatusaccording to claim 5 in which the object is a patient.
 16. Theradiographic apparatus according to claim 12 in which the cyclicalvariation is the patient's breathing cycle.
 17. The radiographicapparatus according to claim 13 in which the cyclical variation is thepatient's breathing cycle.
 18. The radiographic apparatus according toclaim 14 in which the cyclical variation is the patient's breathingcycle.
 19. The radiographic apparatus according to claim 15 in which thecyclical variation is the patient's breathing cycle.